Plate Tectonics
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

 Triple Junctions   Spreading Ridges 

The earth's crust is divided into a number of plates, which drift apart, slide past, or collide with each other, causing the formation, breakup, or merging of continents, and causing volcanism, mountain building, and earthquakes.

The Earth's tectonic plates courtesy of the U.S. Geological Survey

Individual plates:

Ancient Plates

	The supercontingent Pagea is probably the most famous of all former continents. More on Pangea.
The breakup of Pangaea that began some 180 million years ago.

Gondwana

Boundaries:

1. Spreading or divergent boundaries. Where the plates drift apart – ocean ridge (as Mid Atlantic Ridge) or rift (as Africa's great rift)
2. Collission or convergent boundaries. Where plates collide. Plate edges may be either oceanic crust or continental crust. So when plates collide, we have only three possibilities: oceanic-oceanic, oceanic-continental, or continental-continental collisions. If oceanic crust collide with continental crust, the denser oceanic crust is subducted under the less dense continental crust (as at the Ring of Fire). If continental crust collide with continental crust they push each other up in a mountain range (like the Himalayas).
3. Transform boundaries or transform fault boundaries. Where slates slide past each other: (example: the San Andreas fault in California, where the Pacific Plate slides by the North American Plate).

Mid Ocean Ridge

Transform fault

A transform fault is where plates slide past each other. Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins. They are generally defined by shallow earthquakes. A few occur on land, for example the San Andreas fault in California.

Subduction

A slab is the oceanic crustal plate that underthrusts the continental plate in a subduction zone and is consumed by the earth's mantle. Illustration from USGS Visual Glossary

Example from Indonesian Islands

The Boxing Day Tsunami 2004 and the Krokatau tsunami of 1883 both started at this subduction zone.

This is an extremely simplified diagram, not to scale and out of any proportions. The eartquake that triggered the boxing day tsunami is marked in red on the map, and the volcanic islands that constituted only one island, Krakatau, before 416 A.D.? when a caldera collapse destroyed the volcano, is shown (as one island) between Sumatra and Java. Sediment scraped off the Australian Plate during its subduction under the Asian plate forms an accretionary prism or wedge. The tops of this "mountain range" are above sea level and show up as islands (I have not placed any of these islands - like the Mentawai Islands mentioned below - at their exact locations, I just put them there to show the principle). The coldness of the subducting plate permits brittle failure, and thereby earthquakes, down to as much as 700 km along the so called Benioff zone. The Benioff Zone is defined as the active seismic zone in a subduction zone. Water escaping from the descending plate is probably the primary cause of volcanic activity at subduction zones, but whatever the reason melting of rocks take place and magma rises to form a magmatic or volcanic arc. Volcanoes at subduction zones can be extremely explosive like Krakatau in Indonesia and like Mount St. Helens (in the American Cascade Range).

Five years ago Ken Wohletz argued that the Dark Ages may have been triggered by volcano-related climate changes in the 6th Century (in the vicinity of 535 A.D.?), and the Krakatau volcano may have been the culprit. See http://www.ees1.lanl.gov/Wohletz/Krakatau.htm.

	Volcanic arc and Asian continent
	Shelf sea (Eurasian plate)
	Accretion wedge
	Islands within accretion wedge area
	Indian Ocean
	Oceanic crust (Australian plate)
	Oceanic lithosphere (Australian plate)

The subduction zone (here shown as a dark blue line) continues to make half a circle around the centre of the Banda Sea, where the plate tectonic situation is much more complex. In short however the Banda Arc is bounded by an inner volcanic arc and an accretionary outer arc of which Timor is the largest island. Olelog on Banda Sea earthquake (on 27 January 2006) at http://my.opera.com/nielsol/blog/show.dml/125760

As the subducting plate slides beneath the upper plate, stress begins to build where the plates meet and the upper plate can deform to create a large structure called a forearc basin. With time this basin, a sort of a bowl-shaped depression, fills with sediment. It appears that the most severe subduction zone earthquakes occur in areas where such sediment-filled basins are found (like the earthquake that triggered the Boxing Day Tsunami in 2004).

See my blog of 30 January 2006.

The Benioff zone

In a subduction zone the earthquake foci normally plot along a dipping plane at an angle of 33 to 60 degrees and this plane is called a Benioff zone. The zone is named after Hugo Benioff, a US seismologist who first described this feature. The Benioff zone extends to a depth of about 700 km.

Example from Sumatra:

To the left a historic seismicity map from Sumatra. (Red line = subduction zone) The star shows the 8.4 earthquake of 14 September 2007. To the right a cross section A-A'. Both images from USGS.

My blogs on the Boxing Day Tsunami and on Krakatau:

Tsunami Concern – Indian Ocean

Posted on Thursday, 8 December 2005, 11:28:18

It could happen again, and sooner than expected. Two tectonic plates, the Australian and the Eurasian plate (or more precisely the Burma subplate), meet just off Sumatra's south-west coast. It was in this boundary zone that the earthquake occurred that triggered the boxing day tsunami now nearly a year ago. That earthquake was, as I am sure you will remember, near the Aceh province, the most northerly part of Sumatra. Prof Sieh, speaking at the American Geophysical Union Fall Meeting (5–9 December 2005, San Francisco, California, USA), says the concern of scientists is now focused on events further south still, to a region known as the Mentawai Islands patch.

BBC News of 7 December 2005 at: http://news.bbc.co.uk/1/hi/sci/tech/4505818.stm
Abstract

Tsunami Warning Systems – again

Posted on Thursday, 13 October 2005, 10:50:02

Analysis of sound waves produced in the Indian Ocean by the Boxing Day Tsunami, 2004, have shown that monitoring stations set up to detect atomic explosions could help predict the path of a tsunami. Such stations were set up to implement the A-bomb test ban treaty, but they could be involved in the new Indian Ocean tsunami warning system, if the commission of the treaty organisation (CTBTO) gives its permission. The results were reported in the journal Geophysical Research Letters. See BBC News at:
http://news.bbc.co.uk/1/hi/sci/tech/4330286.stm
Tsunami is a Japanese word for a sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands. It is NOT a tidal wave. Tidal waves are caused by the forces of the moon, sun, and planets upon the tides, as well as the wind as it moves over the water.
Here you will find some good tsunami links for more information about tsunamis and of course the existing warning system for the Pacific: http://earthquake.usgs.gov/bytopic/tsunami.html
2004 produced 7 tsunamis, so they are not really seldom. The historically most renowned is probably the Krakatau Tsunami (Indonesia) in 1883. The largest wave reached heights of 40 meters above sea level and killed over 34,000 people. That tsunami was caused by a volcanic eruption (the Krakatau Volcano collapsed).
Ole

Posted on Monday, 14 November 2005, 20:29:44

Krakatau tsunami

One of the greatest natural disasters in modern history was the eruption of the Krakatau (or Kakatoa) volcano in 1883. The following tsunami killed 36,000 people. If a similar explosion of the same power happened today, the tsunami might kill about one million people.

Anak Krakatau, Krakatau's child, was born about 70 years ago and now grows six meters a year. If its growth rate doesn't decrease, the world will face another great tragedy, says Indonesian volcanologist Supriyatman Sutavijaya from the Indonesian University of Volcanology and Geology. If the processes in the magma chamber maintains the current speed, another explosion will happen in 140 years.

Pravda at http://english.pravda.ru/science/19/94/377/16448_Krakatau.html
SFGate.com: http://www.sfgate.com/cgi-bin/article.cgi?f=/news/archive/2005/01/24/financial1017EST0041.DTL

Ole

Other logs on subduction

Subduction

Sunday, 1 January 2006, 12:52:01

On diagrams showing the principles of plate tectonics subduction is usually illustrated as oceanic crust sliding under continental crust at an ocean/continent collision boundary. See my simplified figure.

I also happen to read newsgroups and a few days ago somebody in the newsgroup: sci.geo.geology logically asked: “Are there any plates sliding under and over each other beneath the continents?”

Here are a few examples of subduction taking place under continents:
The Juan De Fuca plate is being subducted beneath the American plate along the northwest margin of the Conntinental U.S.
(See my Blog on Parent found for orphan tsunami)

The Northern portion of the Pacific plate is being subducted beneath the North American plate along the southern margin of Alaska and the Aleutian Islands.

The Nazca plate is being subducted beneath the South American plate.

There are others as well. .......

PS: Such as Iran where the Arabian plate is being subducted beneath the Eurasian plate. (See blog of 1 April 2006)

The granite laccolith in Tierra del Fuego, Chile (person in foreground for scale):

The light-colored rock which makes up the bulk of the mountains is a Miocene granitic laccolith intruded into dark-colored Cretaceous marine rocks

The envelope of the granite laccolith is mainly composed of late Jurassic–early Cretaceous sedimentary and volcanic rocks formed in an island arc-marginal basin system. Folding of the marginal basin infill, which has been related to the closure of the marginal basin, occurred from about 100 to 93 Ma ago. The granite laccolith was emplaced only some 12 million years ago.

Triple junctions:

Rifting often start at a triple junction. See my log of Sunday, 9. October 2005, 09:17:01 Triple junction and rifting.

The rifting continues in the Red Sea and the African Great Rift Valley

More on Triple Junctions

Earthquakes

Most eartquakes are related to plate tectonics and most of them occur near plate boundaries - including mid ocean ridges, see my log of Sunday, 27. November 2005, 15:57:36 on Forgotten Earthquakes

Subduction zone plate boundaries produce three types of earthquakes:

Real subduction earthquakes, which occur due to differential movement (co-seismic slip) at the interface between the oceanic plate and the continental plate. These earthquakes occur offshore, they are relatively shallow (typically less than 30 km deep), and can be of magnitude 8 or greater. Shallow (less than 30 km deep), crustal, intraplate earthquakes. These earthquakes, which are caused by faults in the continental plate, can exceed magnitude 7. Deep, intraplate earthquakes, caused by bending fracture of the subducting oceanic plate. These earthquakes can reach magnitudes 7-7.5.

Volcanoes

Most volcanoes are related to plate tectonics and most of them occur near plate boundaries. The most explosive occur near subduction zones.

The Taconic Orogeny - Circa 450 Million Years Ago

The Taconic orogeny (mountain building) occurred by the Late Ordovician when the Taconian island arc (volcanic island arc) collided with eastern North America along an east dipping subduction zone.

Because subduction dipped eastward it acted like a ramp, and the terrane was forced to slide up onto and over the continental edge forming the Taconic mountains.

See my blog on Mountain Building and Global Cooling about possible climate consequences.

Retroarc systems

Retroarc foreland basins form as a result of lithospheric loading behind a mountainous arc under a compressional regime; they are commonly filled with continental deposits.

The relative rates of plate convergence and subduction is a governing factor. If convergence is faster than subduction some of the shortening is taken up in the over-riding plate to form a retroarc foreland basin. If convergence is slower than subduction at the trench the upper plate is in net extension and an extensional backarc basin forms.A balance between the two results in neither compression nor extension and a 'neutral arc-trench system.

In contrast to peripheral foreland basins, retroarc forelands are situated on continental plates above subducting slabs.

A complex, retroarc foreland basin system is present along the north-eastern and northern side of the Sumatra–Java magmatic arc and associated fold-thrust belts.

Taranaki retroarc foreland basin in New Zealand.

Individual plates treated in Olelog

The Antarctic Plate - Wednesday, 14 December 2005.

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